Many varied attempts have been made previously to formulate and construct a safe sun-screen which would encourage protective suntanning but these have been generally ineffective because they are specifically designed to remove all, or substantially all of the entire short wave UV-B (290-320 nm) either by employing chemical absorbers (e.g. Gallowhur U.S. Pat. No. 2,391,959; Solvay et Cie, French Pat. No. 2,236,195, Sear U.K. Pat. No. 2,097,810, Mutzhas U.S. Pat. No. 4,529,269), by mechanical processing of thermoplastic resins or yarns so as to impart UV-B filtering characteristics (e.g. Solvay et Cie, French Pat. No. 7,324,647; Stotzer, French Pat. No. 8,020,161; Mariac, French Pat. No. 930,621; Glaser, West German Pat. No. 3,101,390) or by including ground glass particles in the support resin and providing multiple spatially arranged laminates each with unique but additive absorption characteristics (e.g. Mutzhas, British Pat. Nos. 1,586,687 and 1,567,979 and U.S. Pat. No. 4,200,360).
However, no previous attempts have been made to preferentially reduce only certain wavelengths in all three categories of dangerous light, which it now seems can interact synergistically to represent a very serious cosmetic and health threatening environment to man, while at the same time providing sufficient transmission of certain wavelengths of UV-B energy within the narrow band 310-320 nm now known to stimulate melanogenesis.
Recognition of the health hazard of UV-radiation has already prompted the U.S. Department of Health Education and Welfare (via NIOSH) to recommend occupational exposure limits to UV radiation including that of natural sunlight (HSM Pub. Nr. 73-11009) and there have also been calls by general medical practitioners for stricter controls on UV-parlors. Also, a recent U.S. Bureau of Food and Drug Administration advisory panel recommended after reviewing available topical sunscreens, that all sun protection products should be placarded with the advisory phrase "Over-exposure to the sun may lead to premature aging of the skin and skin cancer. The liberal and regular use of this product may reduce the chance of premature aging of the skin and skin cancer".
In addition, safety plastic or glass filters have been formulated and used for eye protection against the very short wavelengths ultra-violet rays (UV-C) artificially produced by electric carbon arc welding equipment. These glasses employ very broad spectrum UV blockers such as acetophenone and pigments or dyes such as the soluble green dyestuff "Filter Blaugrun" as visible light filters (British Pat. No. 1,060,780). Such materials are totally unsuitable for sunbathing because they entirely block the harmless UV-A light necessary for immediate pigment darkening as well as the small proportion of UV-B within the wavelength range 310-320 nm which is essential for melanogenesis.
It can be seen therefore that the disadvantages of these earlier radiation screens are that they are either prohobitively expensive to produce, overprotect the sunbather from the UV-B sunburning and melanogenic rays, do not protect from the overheating infra-red rays or they block out all the UV-light and prevent tanning altogether. None of the prior art inventions relating to applied sunscreens are able to protect the sunbather from dangerous intense visible or infra-red light which is now thought to be potentially as dangerous as excessive short wave band (290-310 nm) UV-B with respect to cancer induction and skin aging, nor do they provide for a mechanism whereby de novo melanin synthesis can be encouraged.